1. Field of the Invention
This invention relates to aluminium and aluminium alloy bodies having corrosion-resistant protective layers and to methods of making such layers. The invention is especially, though not exclusively, applicable to such bodies in the form of thin aluminium or aluminium alloy layers, and the coating provided by the invention particularly improves the moisture resistance of such thin layers. Thus the invention is particularly useful when applied to thin aluminium or aluminium alloy layers in optical information recording members and to electronic devices having thin wiring or electrode layers formed of aluminium or aluminium alloy.
2. Description of the Prior Art
It has long been known that aluminium is protected against corrosion, e.g. by water, by the formation of a corrosion-resistant alumina layer at its surface. Such a layer is formed by exposure to air. In a structure used in a building, for example an aluminium window or roof, this layer has typically a thickness of 100 .mu.m and provides excellent corrosion resistance. Obviously, a layer of this kind cannot be used to protect a very thin aluminium body, particularly a thin aluminium layer of thickness of the order of 1-10 .mu.m or less, such as is nowadays used in optical information-storage discs and electronic devices.
The surface treatment of aluminium by electrolytic treatment in organic and inorganic salt solutions has long been known and involves oxidation of the aluminium per se. It is possible to improve the corrosion resistance, if an oxide coating on the surface of aluminium can be formed in larger thickness by a surface treatment technique, but the thickness of the aluminium restricts this. Further, addition of an organic reagent to an electrolytic bath enables a further improvement in the corrosion resistance reliability of aluminium. This is because a porous coating formed by an electrolytic treatment exhibits a remarkable activity by virtue of its very large surface area and adsorbs an organic reagent to form a stable protective coating.
One method of surface treatment through electrolytic treatment in a solution containing an organic chelating agent is described in JP-A-62-80294. 8-hydroxyquinoline (oxine) is added to an aqueous sulphuric acid solution. This is taken to be strong acid solution (pH&lt;1). The articles treated according to this document are building materials, e.g. windows, and the oxide layer produced will have a thickness of more than 100 .mu.m, as in conventional oxidation.
U.S. Pat. No. 4563253 is concerned with making corrosion-inhibited metal by first forming a thin oxide coating on the metal and then electrolytically treating the oxidized metal in a bath containing an organic complexing compound. Many chelating compounds, e.g. oxine, are disclosed. The process is especially applied to iron and steel substrates. Other metals mentioned are zinc and copper. The complexing compound is said to react with the oxide coating and form an insoluble and chemically stable complex with the underlying metal. The pH of the baths containing the complexing compound is typically 8.4, though a pH range of 7-10 is mentioned. The baths contain borate ions. The present inventors have found that with aluminium, at such pH levels and with borate ions present, highly dense, bright thin layers are not produced.